That's the story being pieced together by a research team led by Richard Firestone of the Lawrence Berkeley National Laboratory in Berkeley, California.

The team had done previous work on a suspected impact that occurred 13,000 years ago. But while looking for evidence of that more recent blast in mammoth tusks, the scientists found traces of the much older event.

"The surprise was the tusks were dating back to 30,000 to 34,000 years ago," Firestone said.

"Nobody had thought of it before. It was serendipitous."

The work was presented this week at the fall meeting of the American Geophysical Union in San Francisco, California.

Tiny "Bullet Holes," Micrometeorites Found

While searching for evidence of the more recent cosmic impact, team member Allen West, an Arizona geophysicist, noticed an odd pattern as he was combing through thousands of ancient tusks.

He found that the top-facing sides of a few tusks were pockmarked by holes 0.08 to 0.11 inch (2 to 3 millimeters) across.

The pockmarks were found on seven mammoth tusks—most likely from near Alaska's Yukon River—and the skull and horns of a Siberian bison (see a map of Alaska).

That the holes were found on only one side of the bones indicated that the impacts came from a single direction, Firestone said.